Medical Reports
Complete Blood Count (CBC) Explained: WBC, RBC, Hemoglobin & Platelets
Understand your Complete Blood Count (CBC) lab test. Learn what abnormal White Blood Cells (WBC), Red Blood Cells (RBC), Hemoglobin, and Platelet counts indicate.
Key takeaways
- A Complete Blood Count (CBC) evaluates three primary cellular lineages: white blood cells (immune), red blood cells (oxygen delivery), and platelets (clotting).
- Elevated White Blood Cells (Leukocytosis) often signal acute bacterial/viral infection, inflammation, or physical stress.
- Low Hemoglobin or Hematocrit (Anemia) indicates reduced oxygen-carrying capacity in the blood.
Core Components of a CBC Test
A Complete Blood Count (CBC) is the single most common blood test ordered globally. It provides a comprehensive snapshot of overall immune strength, oxygen transport, and clotting capacity.
Upload your report to our free medical report analyzer for an instant plain-language summary.
- •White Blood Cells (WBC): Normal 4,500 - 11,000 cells/µL. Fights bacterial, viral, and fungal infections.
- •Red Blood Cells (RBC): Normal 4.3 - 5.9 million cells/µL (men) / 3.8 - 5.2 million (women). Delivers oxygen from lungs to body tissues.
- •Hemoglobin (Hgb): Normal 13.5 - 17.5 g/dL (men) / 12.0 - 15.5 g/dL (women). The iron-rich protein that binds oxygen.
- •Platelets (Plt): Normal 150,000 - 450,000 /µL. Small cell fragments essential for blood coagulation and wound repair.
Understanding WBC Differential Counts
A CBC with differential breaks white blood cells into 5 specific subtypes, identifying the exact type of immune response taking place.
- •Neutrophils (50-70%): First responders to bacterial infections.
- •Lymphocytes (20-40%): Key defenders against viral infections (T-cells and B-cells).
- •Monocytes (2-8%): Cleans up damaged tissue and chronic inflammation.
- •Eosinophils (1-4%): Active during allergic reactions and parasitic infections.
- •Basophils (0.5-1%): Releases histamine during immediate allergic responses.
What Abnormal CBC Results Mean
Slight fluctuations outside reference ranges are common and often temporary due to hydration, exercise, or minor colds.
- •Low Platelets (Thrombocytopenia): Can cause easy bruising or bleeding; caused by meds, immune issues, or viral recovery.
- •High Hematocrit (Polycythemia): Indicates elevated blood concentration, commonly caused by dehydration or high altitude.
- •Low RBC / Hgb (Anemia): Causes fatigue and shortness of breath; iron or vitamin B12 deficiency is the primary cause.
Common questions
What causes high white blood cell count on a CBC?
High WBC is most commonly caused by an active bacterial or viral infection, systemic inflammation, severe emotional or physical stress, or certain medications like corticosteroids.
Why are my platelets low on my blood test?
Low platelets can occur post-viral infection, from vitamin deficiencies, alcohol consumption, or auto-immune destruction. Consult your doctor if levels fall below 100,000/µL.